Negative corona device with means for producing a repelling electrostatic field



Nov. 17, 1970 0TH 3,541,329

W. R NEGATIVE CORONA DEVICE WITH MEANS FOR PRODUCING A REPELLINGELECTROSTATIC FIELD Filed Dec. 1, 1966 FIG. Fla. 2

FIG. 3 FIG. 4

INVENTOR.

WALTER ROTH United States Patent US. Cl. 250-495 7 Claims ABSTRACT OFTHE DISCLOSURE Apparatus for depositing uniform negative charge on asurface and including an electrode positioned on the opposite side of acorona wire from the surface and operated at a negative potential equalto or higher than that on the corona wire.

In general, the present invention relates to a discharge device and morespecifically to a negative corona charging device.

It is well known that corona charging devices find numerous applicationsin xerography, electrostatic coating, and other electrostatic processes.In electrostatic imaging processes it is commonly necessary to charge aninsulator or photoconductor with a uniform positive or negative chargepattern. The distinction between positive and negative corona isparticularly significant in the prior art because each of these coronaprocesses appears to proceed by a separate and distinct mechanism whichhas a significant effect upon the nature of the discharge and theresults of the charging of an insulating surface.

A brief discussion of the mechanism of the positive and negative coronadischarge phenomena is necessary to provide an understanding of theprior art problems which the present invention solves.

As a result of naturally occurring ionization processes (cosmic raybombardment), a small number of free electrons and positive ions arenormally present in the air. When a sufiiciently high positive potentialis applied to a conductive wire, the surrounding free electrons arecaused to move toward the wire with sufiicient velocity to ionize someof the neutral gas molecules which they may strike while travelingtowards the positive wire. In this manner additional positive ions andelectrons are produced. The newly createal electrons are themselvesaccelerated towards the corona wire and may in the process of theirtravel collide with other neutral gas molecules producing still moreions and electrons. As a result of the above avalanching process thewire becomes surrounded by a sheath of electrons and positive ions. Thepositive ions are repelled by the positive potential on the corona wire.Some of these positive ions will strike a nearby insulating surfacehaving a ground plane behind it thereby giving the surface a positiveelectrostatic charge. In the above process the corona wire itself playsessentially no part in the corona generating process other thanproviding the necessary electric field. Variations in wire diameterwill, of course, according to the laws of electrostatics, vary thesurrounding electric field strength and thus the properties of thecorona discharge. Isolated points or other surface imperfections in thewire will create locally high electric fields near the wire. However,these points produce field anomalies which have their primary effectclose to the wire surface, while the corona generating process occurs ina sheath extending relatively some distance from the wire. The abovedescription of positive corona is supported by the observation of abluish-white sheath over the entire surface of the wire of uniformintensity. Because the positive corona produced is relativelyindependent of the exact nature of the corona wire by which it isgenerated, it is Patented Nov. 17, 1970 possible to get relativelyuniform positive corona emission from a wire of commercial grade.

The mechansm of negative corona is apparently entirely different. Therate and pattern of electron emission in negative corona is observed tobe very much a characteristic of the wire material and the exact stateof the wire surface. Such factors as dirt spots, areas of oxidation,variations in the crystal structure of the wire, the degree ofsmoothness, and the like have been observed to have pronounced effectson the uniformity of negative corona. Whatever the true causes ortheoretical explanations, it is observed that, as the negative voltageon a small wire is increased, corona discharge commences and thereappears discontinuous, discrete, and approximately periodic lightemission points seen as reddish tufts of glowing gas at points along thewire. On a polished conductor, these glowing points are approximatelyuniformly spaced along the wire. As the voltage is further increased theglowing points on a negative corona wire move closer and closertogether, and their number increases with the current, however in therange of practical potentials the corona never becomes adequatelyuniform. The use of higher voltage is undesirable because of increasedozone production. An increase in ozone is objectionable since it istoxic. causes damage to other components of a copy apparatus such asrubber belts and the like, and, in general, acts as a strong oxidizer.Further, higher voltages create the problem of potential damage to thematerial being charged by high energy electrons or ion impact.

Thus, in contrast to the continuous, uniform glow of positive corona,the glow of negative corona is inherently non-uniform, consisting ofdiscontinuous, discrete, approximately periodic glows. The nonuniformityof negative corona may be reduced somewhat by operating the corona wireat a potential well above that at which negative corona commences and byusing an extremely clean, smooth, uniform wire or a wire uniformlypitted by a process such as sandblasting; however, it is virtuallyimpossible to eliminate the effects of the discrete glow pointssufliciently for the uniform charging of, for example. a xerographicplate by any of the above methods.

There have been numerous attempts to overcome the problem ofnonuniformity in the negative corona. For example, it has been suggestedthat if the distance between the plate to be charged and the corona wireis increased sufficiently there will be the equivalent of a defocusingor radial spreading effect and thus the plate will be charged moreuniformly. This procedure results in some loss of efficiency and imposessevere limitations on the charging speeds obtainable. Further as thedistance is increased, higher voltages are required and dielectricbreakdown becomes a serious problem. Others have suggested that thecorona wire may be sandblasted or twisted into ropes to provide aplurality of many emitting points. The use of radioactive sources hasbeen suggested to lower the voltages at which ionization takes place,thus providing a shift in the relative control voltages. The addition ofan AC signal applied in series and superimposed on a high negative DCcharging voltage has been attempted to eliminate the nonuniformity innegative corona. It has also been proposed that the wire be oscillatedor rotated while it is charged and producing the negative corona tocounteract the effect of the nonuniformity of the negative corona.However, such methods become prohibitive for mechanical reasons ashigher charging speeds are desired. Similarly, the air or gassurrounding the corona wire may be moved or vibrated so as toeffectively alter the fiow of ions and electrons from a particularconductor area. While each of these prior art methods has had somelimited success each has had a corresponding disadvantage in efficiencyor complexity.

Because of the problem of the nonuniformity of the negative corona theprior art has tended to concentrate its efforts on the solution of theless difficult problems related to positive corona. In order to controlthe positive corona to a high degree electrostatic shields andelectrodes have been employed, however these elements have been directedtoward the control of the electrons and ions at some distance after theyhave left the corona wire. Since the prior art electrodes and shieldsare generally directed toward the solution of problems associated withpositive corona they have neither the proper potential or configurationto establish a suitable field in the vicinity of a negative corona wireas it is proposed in the present invention. Indeed, the prior art hasfailed to recognize the problem toward which the present invention isdirected, namely, the establishment of electrostatic conditions in thevicinity of the negative corona Wire which will allow the glow dischargepoints to move closer together at a .given potential, thus improvinguniformity of charging.

Accordingly it is an object of this invention to provide a new, simple,and highly efiicient device and method for correcting negative coronanonuniformity which overcomes the deficiencies of the prior art asdescribed above.

It is a further object of this invention to provide a negative coronadevice capable of greater uniformity than heretofore obtained.

Another object of this invention is to establish electromagnetic fieldconditions favorable for the uniform deposition of negative coronacharges.

It is an additional object of this invention to provide for greateruniformity in the deposited charge on a surface to be charged.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims taken in conjunctionwith the accompanying drawings.

The present invention overcomes the deficiencies of the prior art andachieves its objectives by providing a suitable field in the vicinity ofthe wire through the use of an electrode plate positioned on theopposite side of the corona wire from the surface to be charged andoperated at a potential relative to the potential of the corona wire'itself, so that the divergence of the effective beam of negativecharges produced by each emission point is reduced, thus allowing theglow discharge or emission points to move closer together at a givenpotential, improving the uniformity of charging produced. The above isaccomplished in particular by the establishment of electromagnetic fieldconditions in the region surrounding the negative corona wire which willhave a focusing effect on the cone of charges produced by each emissionpoint along the negative corona wire.

In order to facilitate understanding of the present invention, referencewill now be made to the appended drawings of a preferred embodiment ofthe present invention. The drawings should not be construed as limitingthe invention but are exemplary only. In the drawings:

FIG. 1 is a schematic representation of the present invention.

FIG. 2 is a cross sectional representation of the apparatus of thepresent invention.

FIG. 3 is a pictorial representation in which 3a represents the glowpattern with corona applied alone and 3b represents'the glow patternwith a suitable field applied.

FIG. 4 is a pictorial representation showing the peripheral negativecharges of the cone of charges emitted at each glow point beingeffectively bent back and attracted to a field plate.

A preferred embodiment of the present invention is shown in FIGS. 1 and2 in which a surface to be charged 4 is shown for purposes ofillustration as composed of a photoconductive insulating layer 6 adheredto a grounded electrically conductive backing support plate 8 such as iscommonly utilized in xerographic processes. This plate 8 serves as aground plane so that charge may be retained on the insulating layer 6.Plate 8 may also be biased on either side of ground with correspondingalterations in the other potentials required. In the alternative thebottom of insulating layer 6 may be subjected to positive corona andplate 8 omitted. The photoconductive insulating layer 6 has sufiicientlygood insulating properties to retain an electrostatic charge for areasonable length of time in a practical system which implies aresistivity of at least on the order of 10 ohm-centimeters, in aconventional xerographic system.

At some distance from the surface to be charged 4 is one or more coronacharging electrodes 10. Corona charging electrodes 10 may be in anystructural configuration which is suitable for the production of corona,such as round wires, a knife edge or the like. The distance of coronaelectrode 10 from the surface to be charged 4 for optimum charging ofsurface 4 is determined by the characteristics of the electrode, itsdimensions, the relative voltages applied, and the environmentalconditions. In the preferred embodiment any suitable noncorrosivematerial having a uniform exterior and capable of corona discharge maybe employed as the corona electrode within a wide range of dimensionsand shapes. A typical corona wire array is composed of a smooth 0.0035inch stainless steel wire mounted at their ends in end walls including apolystyrene insulating support block (not shown) or other goodinsulating material with a wire-towire spacing of 0.5 inch in a singleplane (if more than one corona wire is employed). The corona wires neednot be made of a particularly good conductor but are neverthelesspreferably made out of a metal for mechanical strength. The minimumdiameter of corona Wire 10 is determined by considerations of mechanicalstrength. The maximum diameter of corona wire 10 is determined by thefact that the voltage required for corona discharge increases withincreasing wire diameter and approaches that required for sparking.Because of the corrosive nature of the corona discharge which formsozone, oxides of nitrogen and in the presence of moisture nitric acid,the corona wire is preferably corrosion resistant.

Within reasonably broad limits the separation between corona wire 10 andthe surface to be charged 4 is not critical, however, a typicalseparation providing satisfactory charging for a stainless steel wire of3.5-mil diameter with a negative potential of 4-11 kilovolts appliedbetween the backing plate 8 and the corona electrode 10 in an airatmosphere is a /2 inch.

As shown in FIG. 2, a negative source of potential 14 is electricallyconnected to corona electrode 10 to provide for the emission of negativecorona. A typical voltage for potential source 14 is 6,000 volts. Therange of potentials appropriate for corona wire dimensions of the orderdiscussed above required to generate a useful corona discharge is fromapproximately 4,000 volts to 11,000 volts, with a preference forpotentials between 6,000 and 8,000 volts. Smaller wires require lowervoltages and larger wires require higher voltages.

On the side of the corona electrode 10 opposite from the surface to becharged 4 is a negatively charged metallic plate, screen or fieldelectrode 12 which serves to produce a suitable field which results in acontinuous emission pattern as compared with the spotty emission patternof the negative corona in the absence of such a suitable field. While anegatively charged metallic plate is referred to throughout as thepreferred embodiment of the structure 12 for producing the field in thevicinity of corona wire 10 any other suitable structure for producing asuitable electromagnetic field in the vicinity of corona electrode 10may be used. For example, parallel strands of conductive wire; a wovenwire screen; a planar, polygonal, cylindrical or other curved continuousconductive member; an apertured grid; a continuous insulating surfacewith appropriately spaced and connected conductors embedded therein; ora plate of any rigid conducting material such as steel or aluminum maybe.

utilized to produce the field in response to the application of anappropriate potential. The geometry of members producing theelectromagnetic field in the vicinity of corona electrode are such thatat the voltage em.- ployed they do not emit corona themselves. Ingeneral, the plate electrode 12 has a width from 10 to 100 times thewire diameter or the smallest dimension of the corona electrode toinsure that the electromagnetic field of the plate electrode 12 is feltat all points of the emitted charge cone. The separation of the plateelectrode 12 from the corona charging electrode 10' is sufficient toavoid dielectric breakdown. This distance depends upon the appliedpotentials. A typical separation distance between plate electrode 12 andcorona electrode 10 is on the order of /2 inch.

A suitable electromagnetic field can be provided by applying a negativepotential from potential source 16 to a metallic palte 12 having anabsolute value equal to or greater than that potential on the negativecorona wire 10. 'For example, a potential source 14 supplyingapproximately a negative 6,000 volts relative to the surface to becharged 4 may be applied to negative corona wire 10. It is preferredthat potential source 16 apply a negative potential betweenapproximately 7,000 and 8,000 volts to metallic plate 12 under theseconditions to establish a sufiicient repelling field to produce acontinuous glow pattern at the surface to be charged 4 although highervoltages up to approximately 11,000 volts but below the level at whichdielectric breakdown occurs and lower voltages down to approximately4,000 volts are also effective. Variations in the applied voltage frompotential source 16 applied to plate 12 will obviously be required toprovide optimum effectiveness in response to alterations in thepotential 14 applied to corona wire 10 as other parameters of the systemare altered.

In operation, when sufficient negative potential 14 is applied to coronawires 10 a corona discharge results consisting of discontinuous,discrete, approximately periodic glows 22 around the corona wire 10.While it is not intended to limit the invention to any specific theoryof operation it is presently believed that the reason for the periodicstructure of the negative corona glows is that emission starts at a highpoint on the wire where the fields are the highest. The resultingnegative charges and related fields retard emission from neighboringpoints along corona electrode 10 which lie within a strong interactiondistance of the initial high field points. The next emitting point musttherefore be the next high field point just outside the stronginteraction distance of the initial high field points. The stronginteraction distance is apparently increased by scattering anddefocusing of electrons and negative ions in the gas. The interactiondistance is minimized by providing a repelling field in the vicinity ofthe corona wire 10 by applying a negative potential equal to or higherthan that on the corona electrode 10 to metallic plate 12. The reductionof the interaction distance apparently occurs, at least in part, becausethe repelling field from plate 12 narrows the radial spread of theemitted electrons as indicated by the dotted lines in FIG. 1. Therepelling field thus decreases the effect of the emitted electrons andnegative charges on neighboring high points and consequently theinteraction distance between glow points 22 on corona wire 10 isdecreased. The reduction of the interaction distance between the glowpoints 22 on corona wire 10 allows the glow points 22 to move closertogether and eventually merge and thus increase the uniformity ofcharging by producing a continuous glow pattern at all distances equalto and greater than the distance to the surface to be charged 4 from thecorona electrode 10. The above described effect is representedpictorially in FIG. 3 in which FIG. 3a shows the charge patternconsisting of a series of discontinuous, discrete, approximatelyperiodic charge patterns 18 at a surface to be charged 4, when thecorona electrode 10 is energized at approximately 6,000 volts without arepelling field. When the repelling field plate 12 was energized atapproximately 7,500 volts a continuous line charge pattern 20 as shownin FIG. 3b was obtained at the surface to be charged 4.

In the alternative it has also been found that the application of arelatively positive field (that is, the application of a negativepotential less than that applied to the corona electrode 10) to fieldplate 12 will produce a similar effect to that described above. Forexample, with approximately 6,000 volts applied to corona electrode 10,the application of a negative potential from approximately 4,000 voltsupward produces a continuous charge pattern. It is believed that theexplanation for this fact is that peripheral negative charges of thecone of charges emitted at each glow point are effectively bent back andattracted to the field plate as shown in FIG. 4. Further, it is believedthat the relatively higher velocities of the charged particles close tothe axis of the emitted cone in the direction normal to the plateprevents any significant spreading of those particles even when theperipheral charges are stripped from the cone of charge by the appliedfield. This process, also, narrows the radial spread of the emittedcharge cone and decreases its effect on neighboring high points.Consequently the interaction distance between glow points is decreasedand they move closer together producing an increase in the uniformity ofcharging obtainable. This process of stripping the charge cone ofunwanted peripheral negative charges by means of a stripping electrodeis dependent upon such factors as the distance of the plate 12 fromelectrode 10 and the relative dimensions of these elements. The width ofplate 12 is typically several orders of magnitude larger than thediameter of electrode 10 to insure an effective field in the area ofelectrode 10. In the special case where the potential on plate 12 andelectrode 10 are the same the relative dimensions of the field providean effective tangential component of force which serves to provide afocusing effect on the emitted cone of charge.

From the above description it is obvious that any other known meanswhich are capable of establishing a suitable field in the vicinity ofnegative corona electrode 10 may be employed in lieu of field plate 12within the scope of the present invention.

Therefore, in operation, an electromagnetic field is applied in thevicinity of the negative corona wire 10 to minimize the interactionbetween adjacent glow points of the negative corona thereby causing theglow points to move closer together and merge; thus, increasing theuniformity of the corona pattern at a surface to be charged. Theinteraction distance between adjacent glow or emission points of thenegative corona is decreased by decreasing the divergence of the cone ofcharges produced at each emission point by either applying a repellingfield, a tangential force on the charges, or by causing the peripheralcharges to 'be directed out of the effective cone by application of anattractive field which is slightly relatively positive with respect tothe corona potential. For the sake of clarity of disclosure andsimplicity of explanation the invention has been herein described abovein terms of the theory of operation as presently understood although itis to be clearly understood that the theory is illustrative only and isnot intended to be interpreted in limitation of the scope of theinvention.

Although a specific preferred embodiment of the invention has beendescribed in the detailed description above, the description is notintended to limit the invention to the particular forms or embodimentsdisclosed herein, since they are to be recognized as illustrative ratherthan restrictive and it will be obvious to those skilled in the art thatthe invention is not so limited. The invention is declared to cover allchanges and modifications of the specific example of the inventionherein disclosed for purposes of illustration, which do not constitutedeparture from the spirit and scope of the invention.

What is claimed is:

1. A corona discharge device for depositing uniform negative electricalcharge on a surface to be charged, said device comprising:

(1) a corona discharge electrode,

(2) first potential means for supplying a negative corona generatingpotential to said electrode,

(3) an electrostatic field electrode producing a repelling electrostaticfield in the vicinity of said corona electrode to focus each of the coneof charges emitted from said corona discharge electrode so as todecrease the interaction distance between said cones of charges, saidelectrostatic field electrode being located on the opposite side of saidcorona discharge electrode from said surface to be charged, and

(4) second potential means for supplying a potential to saidelectrostatic field electrode of a magnitude at least as high as thatapplied to said corona electrode.

2. The device of claim 1 wherein said corona electrode is approximatelyequal distance between said surface to be charged and said electrostaticfield electrode.

3. The device of claim 1 wherein said electrostatic field electrode hasan effective extent several orders of magnitude greater than thediameter of said corona electrode.

4. The device of claim 1 wherein said electrostatic field electrode isbiased negatively in the range of from about 4-11 kilovolts.

5. The device as set forth in claim 1 wherein said electrostatic fieldelectrode has a width at least several times the smallest dimension ofsaid corona electrode.

6. The device of claim 1 wherein said repelling electrostatic fieldelectrode is a conductive metallic plate and maintained at a negativepotential having an absolute value not less than said negative potentialmaintained on said corona electrode.

7. The device of claim 6 wherein said corona generating potential isapproximately 6,000 volts negative potential and said repellingelectrostatic field is produced by maintaining said electrostatic fieldelectrode at a negative potential greater than 7,000 volts.

References Cited UNITED STATES PATENTS 2,856,533 10/1958 Rosenthal250-495 3,075,078 1/1963 Olden 250-495 WILLIAM F. LINDQUIST, PrimaryExaminer U.S. Cl. X.R.

